Method and apparatus for separating nitrogen and hydrocarbons by fractionation using the fluids-in-process for condenser and reboiler duty

ABSTRACT

System for the separation of gases, particularly of mixtures of a low-boiling hydrocarbon and nitrogen, e.g., a mixture of methane and nitrogen, which involves, according to one embodiment, providing a mixture of such low-boiling hydrocarbon and nitrogen at relatively high pressure, e.g., about 800 p.s.i., cooling the compressed gas mixture approximately to its saturation temperature, passing the cooled compressed mixture in heat exchange relation along the lower portion of a fractionating column operating at a pressure substantially lower than the pressure of the compressed gas mixture, and providing reboil heat to the column, reducing the pressure of the compressed mixture approximately to the pressure in the column, introducing the resulting mixture as feed into the fractionating column, withdrawing nitrogen as overhead from the column, and withdrawing the low-boiling hydrocarbon in substantially pure liquid form from the lower portion of the column, passing overhead nitrogen in heat exchange relation along the upper portion of the column, work expanding the exiting nitrogen and recycling the expanded nitrogen again in heat exchange relation along the upper portion of the column, to provide condensing duty in the column, throttling a portion of the liquid hydrocarbon withdrawn from the column and passing the throttled hydrocarbon in heat exchange relation along the upper portion of the column to provide the balance of the condensing duty required for effecting the separation in the column, passing the exiting vaporized methane and the main portion of the liquid methane withdrawn from the column in heat exchange relation with the compressed feed mixture for cooling same, and compressing the methane product to desired pressure, and passing nitrogen withdrawn from heat exchange relation with the compressed gas mixture for cooling same, the separation of the gas mixture in said column being carried out by nonadiabatic differential distillation and thus increasing the efficiency of the system.

United States Patent lnventor Michael L. Hoffman Beverly Hills, Calif. Appl. No. 674,985 Filed Oct. 12, 1967 Patented June 29, 1971 Assignee McDonnell Douglas Corporation Santa Monica, Calif.

METHOD AND APPARATUS FOR SEPARATING NITROGEN AND HYDROCARBONS BY FRACTIONATION USING THE FLUIDS'IN- PROCESS FOR CONDENSER AND REBOILER DUTY c s Drawing FigS of the compressed mixture approximately to the pressure in the column, introducing the resulting mixture as feed into the [52] U.S. Cl 62/23, fractionating column, withdrawing nitrogen as overhead f 62,31 62/39 62/40' 92/41 the column, and withdrawing the low-boiling hydrocarbon in [5]] Int. Cl F] 3/08 Substantially pure liquid form from the lower portion of the [50] Field of Search 62/31, 39, column passing overhead nitrogen in heat exchange relation 7 along the upper portion of the column, work expanding the [56] Reierences Cited exiting nitrogen and recycling the expanded nitrogen again in heat exchange relation along the upper portion of the column, UNITED STATES PATENTS to provide condensing duty in the column, throttling a portion 1,961,201 4 D B frfl 2/ l of the liquid hydrocarbon withdrawn from the column and 2, 6/1933 P lli z r 2/3 passing the throttled hydrocarbon in heat exchange relation 2,503,265 4/1950 Haynes. /2 along the upper portion of the column. to provide the balance 2,552,451 5/1951 Patterson 62/31 of the condensing duty required for efiecting the separation in 2,658,360 11/1953 Mill 2/ l the column, passing the exiting vaporized methane and the 9 5/1954 M r 2/ 1 main portion of the liquid methane withdrawn from the 2,713,780 7/1955 Williams. 62/31 column in heat exchange relation with the compressed feed 2,713,781 7/1955 Williams 62/31 mixture for cooling same, and compressing the methane 2,823,523 2/1958 Eakin et al 62/39 product to desired pressure, and passing nitrogen withdrawn 3,209,548 10/1965 Grunberg et al 62/29 from heat exchange relation with the compressed gas mixture 3,264,831 8/1966 Jakob 62/39 for cooling same, the separation of the gas mixture in said 3,394,555 7/1968 La Fleur... 62/29 column being carried out by nonadiabatic difi'erential distilla- 3,398,545 8/1968 Nelson et a1. 62/40 tion and thus increasing the efficiency of the system.

PRODUCT S2 N 30 ,7 G3 32% 119i so ExcHANeEQ V ums 1-q/VVVWV /V- 4 8 46 ---Nv\M/w- Primary Examiner-Wilbur L. Bascomb,1r. Attorney-Max Geldin ABSTRACT: System for the separation of gases, particularly of mixtures of a low-boiling hydrocarbon and nitrogen, e.g., a mixture of methane and nitrogen, which involves, according to one embodiment, providing a mixture of such low-boiling hydrocarbon and nitrogen at relatively high pressure, e.g., about 800 p.s.i., cooling the compressed gas mixture approximatcly to its saturation temperature, passing the cooled compressed mixture in heat exchange relation along the lower portion of a fractionating column operating at a pressure substantially lower than the pressure of the compressed gas mixture, and providing reboil heat to the column, reducing the pressure PATENTEIJ JUN29 1971 3,589,137,

SHEET 3 up 3 M/CHQEL L. HOFFMAA/ INVIZN'I'OR.

13y M70 W METHOD AND APPARATUS FOR SEPARATING NITROGEN AND HYDROCARBONS BY FRACTIONATRON USING THE lFLlJlDS-llN-PROCESS FOR CONDENSER AND REBOILIER DUTY This invention relates to the separation of the components of gas mixtures, particularly mixtures of nitrogen and a low boiling-hydrocarbon such as methane, ethane, or propane, by rectification, and is particularly concerned with procedure for the separation of methane in substantially pure form from mixtures thereof with nitrogen, employing in the separation zone the principles of nonadiabatic differential distillation, and with a system for carrying out such procedure.

In the operation of a fractionating column, e.g., for the separation of oxygen and nitrogen from air, it has been found that the liquid and vapor in the column are near or at equilibrium only at certain points in the column. It has been found according to the invention described in the copending applica tion, Ser. No. 539,840, of James D. Yearout, filed Apr. 4, 1966, now abandoned in favor of continuation application Ser. No. 828,806, filed May 26, 1969, that substantially greater efl'rciency is achieved and equilibrium between liquid and vapor is obtained substantially incrementally throughout the column, by adding heat along the lower portion of the column, and by removing heat along the upper portion of the column. Such continuous incremental addition of heat to the lower portion of the column and continuous incremental removal of heat from the upper portion of the column results in differential distillation, rather than fractional" distillation. In this manner, equilibrium is more closely approached throughout the column.

The processing or separation of the components of a gas mixture containing low-boiling hydrocarbon and nitrogen, such as for example mixtures of a major portion of methane and a minor portion of nitrogen, to recover substantially pure hydrocarbon, such as methane, for use as a low-cost fuel which can be readily transported by pipeline and stored, has assumed considerable importance. By the term hydrocarbons or methane in substantially pure form" as employed herein, is intended to denote such hydrocarbons or methane of at least 95 percent purity, and preferably of the order of 97 percent or greater. In such separation operations, it is often desirable to provide a feed gas mixture to be separated which is at a pressure higher than the critical pressure of either component of the mixture, e.g., nitrogen or methane, or any mixtures thereof, and to recover the less volatile component, that is the hydrocarbon or methane component, at a pressure at or near the feed mixture pressure,

Accordingly, the process and system of the invention are particularly designed for the separation of low-boiling hydrocarbon such as methane, and nitrogen, from a mixture thereof, wherein the separation of the gases is carried out by a nonadiabatic differential distillation, to recover the hydrocarbon in substantially pure form, and particularly wherein the feed mixture is at the above noted high pressure and the hydrocarbon component is recovered close to or approximately at the feed gas pressure.

According to the present invention, there is thus provided a process for the separation of a mixture of gases consisting essentially of a low-boiling hydrocarbon, e.g., methane, and nitrogen, which comprises the steps of cooling a compressed feed mixture of the gases approximately to its saturation temperature, passing the cooled compressed mixture in heat exchange relation along the lower portion of a fractionating column to provide reboil heat thereto, said column operating at a pressure substantially lower than the pressure of the compressed gas mixture, reducing the pressure of said compressed mixture approximately to the pressure in the said column, introducing the last-mentioned mixture as feed into said fractionating column intermediate the ends thereof, effecting a separation of said mixture in the column into a nitrogen fraction and a hydrocarbon fraction, withdrawing nitrogen as overhead from the column, passing the overhead nitrogen in heat exchange relation along the upper portions of the column to provide condensing duty therefor, work expanding the exiting nitrogen and further cooling same, recycling said expanded and cooled nitrogen in heat exchange relation along the upper portion of the column to provide additional condensing duty therefor, withdrawing the low-boiling hydrocarbon in substantially pure liquid form from the lower portion of the column and providing further condensing duty in the upper portion of the column required to effect such separation of the mixture therein.

According to one preferred embodiment, a portion of the low boiling hydrocarbon liquid, e.g., liquid methane, withdrawn from the lower portion of the column is separated and cooled, as by throttling to a pressure and temperature permitting vaporization of the throttled methane in the upper portion of the column, and such cooled hydrocarbon or methane is passed in heat exchange relation along the upper portion of the column to supply the additional condensing duty required to effect the separation of the feed mixture in the column.

According to another embodiment, under certain operating conditions, a portion of the overhead nitrogen from the column which has been work expanded can be passed in heat exchange relation along the upper portion of the column to supply the additional condensing duty required to effect the separation of the feed mixture in the column.

According to another embodiment, under certain operating conditions, a portion of the overhead nitrogen from the column which has been work expanded can be passed in heat exchange relation with a portion of the liquid hydrocarbon or liquid methane withdrawn from the bottom of the column, to subcool such liquid methane, followed by throttling thereof and passage of the throttled methane along the upper portion of the column as previously noted, to provide additional condensing duty therein.

According to a further embodiment of the invention, instead of throttling a portion of .the liquid hydrocarbon or liquid methane withdrawn from the column and employing the I throttled methane to provide the required additional condensing duty in the upper portion of the column, there can be employed a separate or external nitrogen refrigeration cycle and wherein compressed and cooled nitrogen in an external closed refrigeration loop is throttled and passed in heat exchange relation with the upper portion of the column, together with the passage of the overhead nitrogen through the upper portion of the column as noted above, to supply the required condensing duty in the upper portion of the column.

in yet another modification of the invention and under certain conditions of operation, following cooling of the compressed feed mixture, such feed can be throttled to some inter mediate pressure prior to entering into heat exchange relation along the lower portion of the column to provide reboil duty, the exiting feed mixture then being throttled approximately to the pressure in the column before being introduced as feed therein, as noted above.

In each of the embodiments of the invention process and system, preferably substantially pure low-boiling hydrocarbon or methane separated in the column is passed in heat exchange relation with compressed feed mixture for cooling same, and the low boiling hydrocarbon or methane is compressed either prior to or subsequent to passage in heat exchange relation with the compressed feed mixture, to deliver low-boiling hydrocarbon or methane at a pressure close to that of the compressed feed mixture. As an illustration, where a compressed feed mixture of 1,000 p.s.i. is employed, substantially pure liquid hydrocarbon or methane product at a pressure of about 800 p.s.i. can be produced.

Also, in preferred practice waste nitrogen from the system is passed in heat exchange relation with compressed feed mixture for cooling same.

The invention will be understood more clearly by the description below of certain preferred embodiments of the invention taken in connection with the accompanying drawing wherein:

FIG. 1 is a schematic representation of a preferred form of separation system for separating substantially pure low-boiling hydrocarbon, erg, methane, from a mixture of such hydrocarbon and nitrogen;

FIG. 2 shows a modification of the system of HO. 1;

' FIG. 3 illustrates another modification of the system shown in FIG. 1;

FIG. 4 is a schematic representation of another modification of the system of FIG. I; and

FIG. 5 is a schematic representation of a further modification of the invention system.

Referring to .FIG. I of the drawing, a compressed natural gas feed mixture containing 70 percent methane and 30 per-. cent nitrogen at a pressure of l,000 p.s.i. (absolute) and at ambient temperature is cooled approximately to a temperature of about l80 K., by passage of such compressed feed gas through coil 12 of a main heat exchangerl4 in countercurrent heat exchange relation with cold product and waste gas streams, as will be described more fully hereinafter.

The resulting compressed feed mixture at 180 K. is introduced at 16 into the lower end of one or more passages 18 of a fractionating unit 20 including a fractionating column 22, and the mixture passing upwardly through the heat exchange passages 18 in indirect heat exchange relation with column 22, provides heat for reboiling in the lower or enriching section 24 of the fractionating column 22. The compressed feed mixture passing through the heat exchange passages 18 is in compressed liquid form and is cooled therein, and the compressed liquid is then throttled at 26 to the pressure in the fractionating column 22, about 300 p.s.i., and reduced in temperature to bubble point temperature, and the throttled mixture is then introduced as feed at an intermediate point indicated at 28, into the column.

The mixture in the column 22 is separated into a low boiling fraction, namely nitrogen, and a higher boiling fraction, that is methane, the nitrogen being withdrawn as overhead at 30 from the top of he column, and substantially pure liquid methane is withdrawn at 32 from the bottom of the column.

The nitrogen overhead fraction withdrawn at 30 is passed through one or more passages 34 in indirect heat exchange relation along the upper portion or rectification section 36 of column 22, to provide reflux and condensing duty for he distillation operation. The warm nitrogen vapor exiting at the lower end of the heat exchange passages 34, and at a temperature of 130 K. and at about 300 p.s.i., is introduced at 38 into the work expander or turbine 40, the nitrogen being expanded therein to a pressure of about 20 p.s.i., and at a reduced temperature of about 80" K. The refrigeration thus produced by the expander 40 in the form of the cooled expanded nitrogen discharge supplied at 42, is passed into one or more passages 44 in indirect heat exchange relation with the stripping section 36 of the column to provide additional condensing duty therein, and the exiting waste nitrogen at 46 is then passed through cell 48 of the main heat exchanger 14 in countercur rent heat exchange relation with compressed feed mixture at 12 for cooling same.

A portion 50, e.g., about l5 percent, of the liquid methane product withdrawn at 32 from the column, at a pressure of about 300 p.s.i. and a temperature of about l6? K. is throttied at 52 to a sufficiently low pressure to condense the lowboiling fraction in the upper portion of column 22, and the throttled methane at a pressure of about p.s.i. and a temperature of about liO K. is passed through one or more passages 54 in indirect heat exchange relation along the upper portion or rectification section as oi the column, and the throttled methane is evaporated and superheated in passages 54 to provide further reflux and condensing duty in the upper portion of the column. The exciting vaporized methane at 56 is then warmed by passage through cell 58 of the main heat exchanger 14 for cooling compressed feed mixture, and the warmed exiting methane at 60 at a temperature slightly less than ambient is compressed at 62 to delivery pressure of about 800 p.s.i., is cooled by passage through coil 61 ofa cooler 63 to about ambient temperature, and delivered at 64.

The remaining or usually major portion of the liquid methane withdrawn at 32 from the column is pumped by means of a liquid pump 66 to delivery pressure of about 800 p.s.i., and the compressed liquid 68 is passed through a coil 70 of the main heat exchanger 14 in countercurrent heat exchange relation with incoming compressed'feed mixture at 12 for cooling same, and the exiting compressed liquid methane at 72 is mixed with the discharge 64 of compressor 62, to provide the compressed methane product at 74.

The portion of the liquid methane which is diverted at 50 to be throttled and used to supply additional reflux in the column, can vary depending upon the composition of the feed mixture. Thus, for example, from about 10 to about SOpercent of the total methane liquid withdrawn at 32 can be so diverted at 50, and usually a minor portion of about 15 to about 30percent of such liquid methane is thus employed for additional condensing duty in the column.

In this system as described above and illustrated in FIG. 1, it is noted that the delivery pressure of the substantially pure methane at 74 is about 800 p.s.i., somewhat below the feed pressure of about 1,000 p.s.i.

In the preferred process system described above and illustrated in FIG. 1 of the drawing, it is noted that the compressed feed mixture passing through heat exchange passages 18, providing reboil duty, and the nitrogen passing through heat exchange passages 34 and 44, and the methane passing through heat exchange passages 54, and providing condensing duty in the column, are brought into heat exchange relation substantially along the length of fractionating column 22, and effecting a nonadiabatic differential distillation throughout the column. In this manner, substantially greater efficiency is achieved and equilibrium between liquid and vapor in obtained substantially incrementally throughout the height of column 22, by adding heat along the lower portion of the column and removing heat along the upper portion of the column. Such continuous incremental addition of heat to the lower portion of the column and continuous incremental removal of heat from the upper portion of the column effected by passing heat exchange fluid along the length of the column as noted above, provides a nonadiabatic differential distillation rather than the conventional fractional distillation, whereby equilibrium is much more closely approached throughout the column, thereby substantially increasing the efficiency of separation in column 22.

Under certain conditions of operation, thesingle stage work expansion at 40 of the warmed overhead nitrogen at 38 may provide a lower temperature than is required for the condencation of reflux in column 22. Under these conditions, as illustrated in FIG. 2 of the drawing, work expansion of such warmed overhead nitrogen at 38 can take place in two or more stages, with reheat being provided by warming the turbine discharges in the heat transfer passages of the rectification section of the column after each work expansion. Thus, the warmed exiting nitrogen at 46' following the passage thereof through passages 44 after the first work expansion at 40, can be work expanded in a second stage by expander 76, and the expanded and further cooled nitrogen introduced at '78 into one or more heat exchange passages 80, and passed therein in heat exchange relation along the upper portion of column 22. to provide additional condensing duty, and the exiting expanded nitrogen at 46 passed in heat exchange relation with compressed feed mixture in the main heat exchanger at 14 as described above with respect to FIG. 1. in the embodiment of FIG. 2, employing two expansions and wherein the expanders or turbines 40 and '76 supply the requisite refrigeration for the system, the first expansion in turbine 40 can reduce the pressure of the nitrogen at 38 from 300 p.s.i. down to about p.s.i. and a temperature of about 1 10 K., and the second expansion in turbine 76 can reduce the pressure of the expanded nitrogen down to about 20 p.s.i. and a temperature of about 110 l(. The system of FIG. 2 is otherwise the same as that shown in FIG. 1.

The heat transfer passages 18 of the enriching section 24 of the column may not, in some cases, be able to withstand the high feed pressure; e.g., of the order of about 1,000 p.s.i. Under these conditions, the feed can be throttled to some intermediate pressure prior to entering the heat transfer passages at 118. However, if partial condensation of the feed takes place at such intermediate pressure, the condensing temperature may be too low to provide adequate reboil for column 22. The system illustrated in FIG. 3 can be employed under these circumstances. Referring to FIG. 3, the high-pressure feed indicated at 16 is cooled by passage through a coil 82 of a reboiler 84 in contact with liquid methane 86 withdrawn at 88 from the bottom of the column, and providing some of the reboil for the enriching section 24 of the column, the vapors passing via line 90 from the reboiler back to the column. The cooled feed leaving the reboiler at 92 is then passed through coil 94 ofa heat exchanger 96 in countercurrent heat exchange relation with the nitrogen vapor streams 38 and 46 and the methane vapor stream 56, passing through coils 98, 99 and 100, respectively, of the heat exchanger 96. The further cooled and compressed feed exiting coil 94 is then throttled at 102 to an intermediate pressure of 500 p.s.i., and a temperature of about 180 K. The throttled and expanded feed is then introduced at 104 into the heat exchange passages 18 and passed therein in heat exchange relation along the lower portion 24 of the column to provide adequate reboil.

As a practical matter, a pressure of about 500 p.s.i. corresponds to about the maximum desirable pressure which can be tolerated in heat exchange passages 18. It is preferred to maintain the pressure in passages 18, however, as high as possible in order to maintain a single high pressure liquid phase for the feed fluid being conducted through these passages, for highest efficiency.

The system of FIG. 3 is otherwise the same as that described and illustrated in FIG. I.

In FIG. 4 there is shown still another modification of the system illustrated in FIG. 1. The system illustrated in FIG. 4 can be employed where, for example, the feed mixture is com posed of a high percentage of nitrogen as compared to lowboiling hydrocarbon; e.g., methane, such as a mixture containing about 80 to about 90 percent nitrogen. Thus, referring to FIG. 4, a portion, e.g., about to about 90 percent, of the nitrogen, following work expansion at 40, can be divided from the main stream 42, and conducted as indicated at 106 through a coil 108 of a heat exchanger 110 in countercurrent heat exchange relation with liquid methane at 50, passing through coil 112 of the heat exchanger. In some cases, according to this embodiment, all of the work-expanded nitrogen at 42 can be conducted through coil 108 for subcooling the liquid methane at 112, instead of employing a portion of such expanded nitrogen for direct condensing duty in passages 44. The subcooled exiting methane at a temperature of about 1 I(., is then throttled at 52 and passed through passages 54 in heat exchange relation along the upper portion of the column to provide additional condensing duty therein as described above with respect to the system of FIG. I. The warmed nitrogen at 114 exiting heat exchanger 110, is then mixed with the waste nitrogen stream 46 at 116, and passed at 48 through the main heat exchanger 14 to cool incoming compressed feed as described above. Thus, it will be seen that in the system of FIG. 4 wherein there is a relatively large amount of overhead nitrogen 30 available from the column, a portion or all of such nitrogen following expansion in the expander 40, can be employed to subcool that portion of methane which is recycled and throttled and vaporized in heat exchange relation to the upper portion of the column to provide adequate reflux and condensing duty.

In FIG. 5 there is shown another modified system according to the invention in which a separate eternal nitrogen refrigeration loop is employed to provide adequate condensing duty for the column, in place of recycling and throttling a portion of the liquid hydrocarbon, e.g., methane product, as described in FIG. 1, for this purpose. Thus, where the initial feed mixture contains substantial proportions of the higher boiling hydrocarbons such as ethane and propane, rendering it difficult to drop the pressure sufficiently by throttling at 52 to provide a low enough temperature to supply condensing duty at 54 in FIG. I, a system of the type illustrated in FIG. 5 is ad vantageous.

Thus, in the system of FIG. 5, it is. noted that all of the hydrocarbon product, which can include, in addition to methane, some of the higher boiling hydrocarbons ethane and propane, withdrawn from the lower portion of the column at 32 is pumped up at 66 to delivery pressure and conducted via coil 70 of the heat exchanger 14 to product delivery at 74. To provide adequate reflux, there is provided an eternal closed nitrogen refrigeration loop wherein nitrogen is compressed at 118 to a pressure of about 1,500 p.s.i., is cooled by passage through coil 120 of the heat exchanger 14, against recycled nitrogen at and compressed hydrocarbon product at 70, and the cold compressed nitrogen at 122 is then throttled at 124 to a pressure of about 300 psi. and reduced in tempera ture to about 1 10 K., and the throttled cold nitrogen at 126 is then passed through the heat exchange passages 54 along the upper portion of the column to provide adequate condensing duty in the column, and the warmed exiting nitrogen at 128 is then recycled by passage through c-oil 1130 of the heat exchanger 14 to the inlet of the compressor 118 for recompression. The system of FIG. 5 is otherwise the same as that described above and illustrated in FIG. 1. of the drawing.

In unit 20 the heat exchange passages or constructions 18, 34, 44, 80 and 54, for passage of the compressed feed, the overhead nitrogen vapor, and the throttled methane or eternal nitrogen refrigerant, as heat exchange fluids in heat exchange relation along the column 22 to effect the above-described nonadiabatic distillation therein, can be in the form of unitary plate-fin heat exchanger (not shown) arranged in indirect heat exchange relation with channels bearing the liquid-vapor mixture being separated in the column 22. Such channels may be constructed in a manner of a series of perforated fins, or plates, producing the effect ofdistillation column trays. This is a known type of heat exchanger arrangement described in International Advances in Cryogenics, Volume 10, 1965. A heat exchanger arrangement or construction of this type is also disclosed in the copending application, Ser. No. 572,135, filed Aug. 12, 1966, of James D. Yearout, now Pat. No. 3,508,412 and which is incorporated herein by reference. Since such heat exchanger arrangements or constructions per se form no part of the present invention, they are not shown herein. Although such a plate-fin type of heat exchanger arrangement is preferably employed, any other suitable form of heat exchanger apparatus can be employed in providing the unit 20 containing the fractionating column 22 positioned in indirect heat exchange relation with the passages 18, 34, 44, 54 and 80, as described above and shown in the drawing, so as to effect the above-described differential distillation in the fractionating column 22.

It will be understood that the system described above including the temperatures and pressures set forth are only illustrative and are not intended as limitative of the invention.

From the foregoing, it is seen that the invention provides a novel method and system for separating the components of a feed mixture containing nitrogen and low-boiling hydrocar bons such as methane, ethane and propane wherein the feed mixture to be separated is at a pressure higher than the critical pressure of the components, and it is desired to recover the hydrocarbon or hydrocarbons at a pressure approximately or close to the feed pressure, such system employing the overhead nitrogen separated in the column for providing a major portion of the condensing duty by means of one or a plurality of work expansions providing refrigeration for the system, and wherein nonadiabatic differential distillation is applied to provide maximum efficiency. Feed mixtures which can be advantageously separated to provide substantially pure hydrocarbon product at high pressure according to the invention, include compositions containing from about 10 to about 60 percent nitrogen and about 40 to 90 percent methane. As previously noted, and as applied in the systems described above and illustrated in the drawings, the system is particularly advantageous for separating substantially pure methane from natural gas mixtures containing about 70 percent methane and about percent nitrogen.

While i have described particular embodiments of my invention forthe purpose of illustration, it should be understood that various additional modifications and adaptations thereof may be made within the spirit of the invention, and within the scope of the appended claims.

lclaim:

l. A process for the separation of a mixture of gases consisting essentially of a low-boiling hydrocarbon and nitrogen, which comprises the steps of cooling a compressed feed mixture of said gases approximately to its saturation temperature, passing said cooled compressed mixture in heat exchange relation upwardly along the lower portion of a fractionating column to provide reboil heat thereto, said column operating at a pressure substantially lower than the pressure of said compressed gas mixture, reducing the pressure of said compressed mixture approximately to the pressure in said column, directly introducing said last-mentioned mixture as feed into said fractionating column intermediate the ends thereof, said cooled compressed mixture passing upwardly in heat exchange relation along the entire lower fractionating portion of said column up to the point of introduction of said feed into said column, effecting a separation of said mixture in said column into a nitrogen fraction and a hydrocarbon fraction, withdrawing nitrogen as overhead from said column, directly passing said overhead nitrogen in heat exchange relation along the upper portion of said column to provide condensing duty therefor, work expanding the exiting nitrogen and further cooling same, recycling said expanded and cooled nitrogen in heat exchange relation along the upper portion of said column to provide additional condensing duty therefor, withdrawing i said low-boiling hydrocarbon in substantially pure liquid form from the lower portion of said column, and providing further condensing duty in the upper portion of said column required to effect said separation of said mixture therein by separating a portion of the liquid hydrocarbon withdrawn from the lower portion of said column, cooling said last-mentioned portion of hydrocarbon, and directly passing said cooled hydrocarbon in heat exchange relation vertically along the upper portion of said frsctionating column, said overhead and expanded nitrogen and said cooled hydrocarbon passing in heat exchange relation along approximately the entire upper fractionating portion of said column, and effecting a nonadiabatic differential distillation along the entire length of said column.

2. A process as defined in claim 1, wherein said low-boiling hydrocarbon is methane.

3. A process as defined in claim 1, wherein said low-boiling hydrocarbon is methane, and wherein said further condensing duty is provided by separating a portion of liquid methane withdrawn from the lower portion of said column, throttling and cooling said separated portion of methane, and passing said throttled and cooled methane in heat exchange relation along the upper portion of said fractionating column.

4. A process as defined in claim 3, and wherein said reduction of the pressure of said compressed feed mixture is accomplished by throttling said compressed mixture after passage thereof in heat exchange relation along the lower portion of said column, to a reduced temperature and to a pressure approximately the pressure in said column.

5. A process as defined in claim ll, wherein said fccd gas mixture consists essentially of about 70 percent methane and about 30 percent nitrogen, said nitrogen-methane feed mix ture being initially compressed to about l,000 p.s.i., and which includes delivering the liquid methane withdrawn from the lower portion of the column as product at pressure of about 800 p.s.i.

6. A process asdefined in claim 1, wherein said expanded nitrogen withdrawn from heat exchange relation from the upper portion of said column is passed in heat exchange relation with said compressed feed gas mixture for cooling same, and wherein said hydrocarbon withdrawn from the lower portion of said column is passed in heat exchange relation with said feed gas mixture for cooling same.

7. A process as defined in claim 3, and including passing the exiting expanded nitrogen recycled in heat exchange relation along said fractionating column, in heat exchange relation with said compressed feed gas mixture, pumping the major portion of liquid methane withdrawn from the lower portion of the column up to desired delivery pressure, and passing said pumped liquid hydrocarbon in heat exchange relation with said feed gas mixture for cooling same, passing the remaining minor portion of methane throttled and vaporized along the upper portion of said main fractionating column into heat exchange relation with said feed gas mixture for cooling same, and compressing said last-mentioned minor portion of methane and mixing same with said major portion of pumped liquid methane and delivering the resulting methane as product at a pressure somewhat below the pressure of feed gas mixture.

8. A process in claim 1, including passing a portion of said work expanded nitrogen into heat exchange relation with a portion of said hydrocarbon withdrawn from the lower portion of the column, to subcool said hydrocarbon, throttling said subcooled hydrocarbon to reduce the pressure and temperature thereof, and passing said throttled hydrocarbon along the upper portion of the fractionating column to supply said further condensing duty therefor.

9. A process as defined in claim 3, wherein a portion of the work-expanded nitrogen is passed in heat exchange relation with a portion of the methane withdrawn from the lower portion of the column, to subcool said liquid methane, prior to throttling of said liquid methane and prior to passage thereof in heat exchange relation along the upper portion of the column, and including passing the exiting methane withdrawn from heat exchange relation with the upper portion of the column, in heat exchange relation with the feed gas mixture, and passing the exiting heated nitrogen into heat exchange relation with the compressed feed gas mixture for cooling same.

it). A process as defined in claim 1, wherein said compressed fecd mixture is first reduced in pressure to an intermediate pressure higher than the pressure in said column prior to entering into heat exchange relation along the lower portion of said column, and after passage in heat exchange relation with said column, the exiting feed gas mixture is then reduced in pressure approximately to the pressure in the column prior to introduction as feed therein.

11. A process as defined in claim 3, wherein said compressed feed gas mixture is first precooled by passage in heat exchange relation with liquid in a reboiler in fluid communication with the lower portion of said column, and said precooled compressed feed gas mixture is then further cooled by passage in heat exchange relation with said nitrogen prior to expansion thereof, and with said expanded nitrogen, and also in heat exchange relation with throttled methane exiting from heat exchange relation with the upper portion of said column, and said further cooled mixture is throttled to an intermediate pressure above the pressure in said column prior to said passing of such cooled compressed mixture in heat exchange relation along the lower portion of said column for providin rcboil heat therein.

12. A process as defined in claim 1, wherein said work-expanded nitrogen exiting from heat exchange relation with the column is further work expanded to a reduced pressure and temperature and the further expanded nitrogen again passed in heat exchange relation along the upper portion of said column for providing additional condensing duty therein.

13. A system for the separation of the components ofa mixture of gases, which comprises a fractionating column, a first passage means for passing a cooled compressed gas mixture in heat exchange relation upwardly along the entire lower portion of said fractionating column, means for reducing the pressure of said gas mixture exiting said first passage means, means for directly introducing the resulting gas mixture as feed into the fractionating column intermediate the ends thereof, to ef feet a separation of said mixture in said column, means for directly recycling fluid from the upper end of said fractionating column downwardly into heat exchange relation along approximately the entire upper portion of said fractionating column, means for work expanding recycled fluid from said last-mentioned means, a second passage means in heat exchange relation along approximately the entire upper portion of said column, for passing said work expanded fluid therethrough, means for withdrawing work-expanded fluid from said last-mentioned means, means for withdrawing fluid product from the lower portion of said fractionating column, means for separating a portion of said last-mentioned fluid, means for reducing the pressure of said last-mentioned portion of fluid, a third passage means for directly passing said last-mentioned portion of fluid in heat exchange relation vertically along approximately the entire upper portion of said column, and means for delivering said last-mentioned portion of fluid and the remainder of the fluid withdrawn from the bottom of said fractionating column under pressure as product.

14. A system as defined in claim 13, including means for conducting a portion of the work expanded fluid into heat exchange relation with said portion of fluid withdrawn from the lower portion of said column, prior to reducing the pressure thereof and prior to introducing same into said third passage means.

15. A system as deflned in claim 1.3, including a main heat exchanger, conduit means for passage of compressed feed gas mixture through said heat exchanger and communicating with the lower end of said first passage means, conduit means connecting the discharge end of said second passage means and passing through said heat exchanger, means for pumping a major portion of fluid withdrawn from the lower end of said fractionating column to a predetermined delivery pressure, means for conducting said last-mentioned major portion of fluid through said heat exchanger, means for passing a minor portion of separated fluid withdrawn from the lower portion of said column and exiting said third passage means through said heat exchanger, and means for compressing the exiting lastmentioned portion of fluid to said predetermined pressure, and for mixing said major and minor portions of fluids, and withdrawing said last-mentioned mixture as product. 

2. A process as defined in claim 1, wherein said low-boiling hydrocarbon is methane.
 3. A process as defined in claim 1, wherein said low-boiling hydrocarbon is methane, and wherein said further condensing duty is provided by separating a portion of liquid methane withdrawn from the lower portion of said column, throttling and cooling said separated portion of methane, and passing said throttled and cooled methane in heat exchange relation along the upper portion of said fractionating column.
 4. A process as defined in claim 3, and wherein said reduction of the pressure of said cOmpressed feed mixture is accomplished by throttling said compressed mixture after passage thereof in heat exchange relation along the lower portion of said column, to a reduced temperature and to a pressure approximately the pressure in said column.
 5. A process as defined in claim 1, wherein said feed gas mixture consists essentially of about 70 percent methane and about 30 percent nitrogen, said nitrogen-methane feed mixture being initially compressed to about 1,000 p.s.i., and which includes delivering the liquid methane withdrawn from the lower portion of the column as product at pressure of about 800 p.s.i.
 6. A process as defined in claim 1, wherein said expanded nitrogen withdrawn from heat exchange relation from the upper portion of said column is passed in heat exchange relation with said compressed feed gas mixture for cooling same, and wherein said hydrocarbon withdrawn from the lower portion of said column is passed in heat exchange relation with said feed gas mixture for cooling same.
 7. A process as defined in claim 3, and including passing the exiting expanded nitrogen recycled in heat exchange relation along said fractionating column, in heat exchange relation with said compressed feed gas mixture, pumping the major portion of liquid methane withdrawn from the lower portion of the column up to desired delivery pressure, and passing said pumped liquid hydrocarbon in heat exchange relation with said feed gas mixture for cooling same, passing the remaining minor portion of methane throttled and vaporized along the upper portion of said main fractionating column into heat exchange relation with said feed gas mixture for cooling same, and compressing said last-mentioned minor portion of methane and mixing same with said major portion of pumped liquid methane and delivering the resulting methane as product at a pressure somewhat below the pressure of feed gas mixture.
 8. A process in claim 1, including passing a portion of said work expanded nitrogen into heat exchange relation with a portion of said hydrocarbon withdrawn from the lower portion of the column, to subcool said hydrocarbon, throttling said subcooled hydrocarbon to reduce the pressure and temperature thereof, and passing said throttled hydrocarbon along the upper portion of the fractionating column to supply said further condensing duty therefor.
 9. A process as defined in claim 3, wherein a portion of the work-expanded nitrogen is passed in heat exchange relation with a portion of the methane withdrawn from the lower portion of the column, to subcool said liquid methane, prior to throttling of said liquid methane and prior to passage thereof in heat exchange relation along the upper portion of the column, and including passing the exiting methane withdrawn from heat exchange relation with the upper portion of the column, in heat exchange relation with the feed gas mixture, and passing the exiting heated nitrogen into heat exchange relation with the compressed feed gas mixture for cooling same.
 10. A process as defined in claim 1, wherein said compressed feed mixture is first reduced in pressure to an intermediate pressure higher than the pressure in said column prior to entering into heat exchange relation along the lower portion of said column, and after passage in heat exchange relation with said column, the exiting feed gas mixture is then reduced in pressure approximately to the pressure in the column prior to introduction as feed therein.
 11. A process as defined in claim 3, wherein said compressed feed gas mixture is first precooled by passage in heat exchange relation with liquid in a reboiler in fluid communication with the lower portion of said column, and said precooled compressed feed gas mixture is then further cooled by passage in heat exchange relation with said nitrogen prior to expansion thereof, and with said expanded nitrogen, and also in heat exchange relation with throttled methane exiting from heat exchange relation with the upper portIon of said column, and said further cooled mixture is throttled to an intermediate pressure above the pressure in said column prior to said passing of such cooled compressed mixture in heat exchange relation along the lower portion of said column for providing reboil heat therein.
 12. A process as defined in claim 1, wherein said work-expanded nitrogen exiting from heat exchange relation with the column is further work expanded to a reduced pressure and temperature and the further expanded nitrogen again passed in heat exchange relation along the upper portion of said column for providing additional condensing duty therein.
 13. A system for the separation of the components of a mixture of gases, which comprises a fractionating column, a first passage means for passing a cooled compressed gas mixture in heat exchange relation upwardly along the entire lower portion of said fractionating column, means for reducing the pressure of said gas mixture exiting said first passage means, means for directly introducing the resulting gas mixture as feed into the fractionating column intermediate the ends thereof, to effect a separation of said mixture in said column, means for directly recycling fluid from the upper end of said fractionating column downwardly into heat exchange relation along approximately the entire upper portion of said fractionating column, means for work expanding recycled fluid from said last-mentioned means, a second passage means in heat exchange relation along approximately the entire upper portion of said column, for passing said work expanded fluid therethrough, means for withdrawing work-expanded fluid from said last-mentioned means, means for withdrawing fluid product from the lower portion of said fractionating column, means for separating a portion of said last-mentioned fluid, means for reducing the pressure of said last-mentioned portion of fluid, a third passage means for directly passing said last-mentioned portion of fluid in heat exchange relation vertically along approximately the entire upper portion of said column, and means for delivering said last-mentioned portion of fluid and the remainder of the fluid withdrawn from the bottom of said fractionating column under pressure as product.
 14. A system as defined in claim 13, including means for conducting a portion of the work expanded fluid into heat exchange relation with said portion of fluid withdrawn from the lower portion of said column, prior to reducing the pressure thereof and prior to introducing same into said third passage means.
 15. A system as defined in claim 13, including a main heat exchanger, conduit means for passage of compressed feed gas mixture through said heat exchanger and communicating with the lower end of said first passage means, conduit means connecting the discharge end of said second passage means and passing through said heat exchanger, means for pumping a major portion of fluid withdrawn from the lower end of said fractionating column to a predetermined delivery pressure, means for conducting said last-mentioned major portion of fluid through said heat exchanger, means for passing a minor portion of separated fluid withdrawn from the lower portion of said column and exiting said third passage means through said heat exchanger, and means for compressing the exiting last-mentioned portion of fluid to said predetermined pressure, and for mixing said major and minor portions of fluids, and withdrawing said last-mentioned mixture as product. 